It has been revealed recently that organic harmful substances such as organic halides including dioxins and aromatic chlorides, which are considered to be their precursors, are contained in the gases which were exhausted from the incinerators for burning out urban garbage and industrial waste or the facilities for gasifying and fusing of the urban garbage and industrial waste. In addition, it is pointed out that organic harmful substances including bromodioxins and polybromodioxins are contained in the exhaust gases through thermal treatment of plastic materials used in home electric appliances waste. Regarding the organic halides, although each of the compounds have different toxicities, they are strongly toxic in general, and in particular dioxins are considered to be deadly poisonous which can strikingly affect the human body, animals and plants by causing deformity or disturbing effects of hormones.
Therefore, the pollution of the environment becomes now a social problem and reduction of the content of organic halides in the exhaust gases from the burning treatment has been urgently required. At present, as removing methods of organic halides including dioxins, various methods such as absorption method using an activated carbon, a thermolysis method or a contact decomposing method using a catalyst are proposed. Among them, the contact decomposing method has excellent characteristics in that it can avoid resynthesis of dioxins because the dioxins can be decomposed under a lower temperature condition below 300° C.; and that, since it does not require the secondary treatment which is required in the absorption method and the like, the running cost is low.
As a catalyst used in the above described contact decomposing method, various metal oxides such as vanadium pentaoxide and titanium dioxide; carbonates; and silicates are known as described in Japanese Patent Publication No. 38863/94. They are, however, not practical, because they require a high temperature for decomposition and long residence time from several minutes to several hours. Vanadium pentaoxide-titanium dioxide catalysts are disclosed in Japanese Patent Laid-Open No. 144117/31 and Japanese Patent Laid-Open No. 117557/96. However, when SO2 is included in the gas to be treated or when SO2 generates through the decomposition treatment, the oxidation activity of SO2 become higher in the use of the catalysts containing vanadium pentaoxide. As a result, there is a problem that a large amount of SO3 is produced when these catalysts are applied for purification of the exhaust gases containing SOX besides NoX.
There is the other problem that reduction of the catalytic activity occurs according to the time progress in their uses. The following reasons for this activity reduction can be mentioned:                1. Regarding the method of reducing a substance to be treated in the presence of a catalyst by using ammonia as a reducing agent, the catalytic activity decreases by accumulation of ammonium salts of sulfur such as ammonium bisulfate on the catalyst surface, which is formed by reaction of ammonia and SO3 generated as above.        
2. There is a further problem that since the oxidizing activity of SO2 increases, vanadium pentaoxide is reduced and transferred to a water-soluble vanadyl sulfate (VOSO4 having α type crystals: α-VOSO4) and, thus, the catalytic activity is reduced according to the time progress in their uses.
3. When chlorides such as HCl and Cl2 including chlorine gas are included in the gas to be treated or when they are generated through the decomposition process, change in the quality occurs and the catalytic activity are reduced.
To solve these problems, the catalyst in which tin dioxide is contained in the vanadium pentaoxide-titanium dioxide type catalyst as described in Japanese Patent Laid-Open No. 318,135/96, and the catalyst containing molybdenium trioxide as described in Japanese Patent Laid-Open No. 117,557/96 are proposed. Further, Japanese Patent Laid-Open No. 386/94, U.S. Pat. No. 5,227,356 and German Patent No. 4,419,974 disclose a catalyst in which alkaline metal sulfates such as barium sulfate are added to vanadium pentaoxide-titanium dioxide. In addition, Japanese Patent Publication No. 2,633,316 discloses a vanadium pentaoxide-titanium dioxide-tungsten trioxide catalyst, which has resistance to impurities such as nitrogen oxides, sulfur oxides and heavy metals contained in a burning exhaust gas. Furthermore, Japanese Patent Laid-Open No. 192,455/97 discloses a catalyst in which the other component is further added to vanadium pentaoxide and titanium dioxide.
However, there has been a case where the catalysts containing vanadium pentaoxide as the main active component may be insufficient in their performances yet in resistance against HCl or SOX, and, therefore, a catalyst of a higher activity and durability, which can be provided in a lower cost, has been required.